1.Field of the Invention
The present invention relates to a balance corrector for scanner motors of an inner rotor type used in scanning optical apparatus provided with a rotary polygon mirror for scanning a light beam on a photosensitive member, and a balance correcting method thereof.
2.Related Background Art
Recently, the scanner motors used in the scanning optical apparatus are demanded to be rotated at high speed or at high accuracy, and particularly in the case of laser beam printers etc., balance correction (balancing) for a rotary member is indispensable in order to attain high-accuracy deflection scanning apparatus. In order to decrease a windage loss or the like to achieve high-speed rotation, an inner rotor type scanner motor with small inertia or the like is frequently used rather than an outer rotor type scanner motor.
A conventional scanner motor of the inner rotor type is shown in FIG. 11. In FIG. 11, a rotary sleeve 22 is rotatably fit around a stationary shaft 21, and these members are made of ceramics. A stationary member 23 made of a metal material is mounted on the outer periphery of the rotary sleeve 22 by shrinkage fitting or the like. A rotary polygon mirror 24 is fixed to the stationary member 23 by a plate spring 25, and a drive magnet 26 is fixed to the stationary member 23 by adhesion or the like. The stationary shaft 21 is fixed in a motor housing 27, and a motor board 29 to which a stator 28 and electric components etc. are mounted is located as opposed to the drive magnet 26, in the motor housing 27, thus constituting a drive motor.
Further, a permanent magnet 30 is placed on the bottom of the rotary sleeve 22, while a permanent magnet 31 is placed on the stationary shaft 21 so as to be vertically opposed to the permanent magnet 30. These permanent magnets 30, 31 support a load in the thrust direction by mutual repulsion. Here, when the drive motor rotates the rotary sleeve 22, an air film is formed between the stationary shaft 21 and the rotary sleeve 22, whereby the rotary sleeve 22 and the rotary polygon mirror 24 can hydrodynamically be rotated in a non-contact manner.
Balance correction is normally carried out as follows. First, an amount of unbalance is measured as rotating the rotary body including the rotary sleeve 22. Then the rotary body including the rotary sleeve is pulled away from the rotary shaft 21. A first balancing step is to deposit a predetermined amount of balancing weight on the top surface of the rotary polygon mirror 24 through a dispenser 156, as shown in FIG. 12A, and then to irradiate the balancing weight with ultraviolet light from an ultraviolet emitter 158 thereby to fix the balancing weight. A next step is to reverse the rotary body, as shown in FIG. 12B, and to fixedly deposit another balancing weight on the bottom surface of the drive magnet 26 in the same manner. After that, the rotary body is again brought into fit on the stationary shaft 21. Then the rotary body is again rotated to measure an amount of unbalance after the first balancing. The above steps are repeated to achieve high-accuracy balancing.
The above conventional example, however, had the following problems. Since the drive magnet is surrounded by the stator coil because of the structure of the inner rotor motor, a hydrodynamic bearing includes the following problems in carrying out balancing of motor on the two surfaces, i.e. on the rotary polygon mirror 24 and on the drive magnet 26: it requires mounting and dismounting of the rotary body relative to the stationary shaft for every balancing, which increases a probability of damaging the bearing part and which requires cleaning of the bearing part every balancing, thus increasing work procedures and lowering productivity.
When a ball bearing or the like, which is not a hydrodynamic bearing, is employed, the rotary body cannot be dismounted alone, thus requiring correction of balance by another method such as belt drive only with the rotary body before assembling. Since balancing cannot be performed by self drive, enhancement of the balancing accuracy cannot be expected.